Abstract: The cooling water requirements of the atomic power plant on the Susquehanna River are 3350 cfs. Periods of low river flow, high natural river water temperatures, and high wet bulb temperatures are considered the most critical times for aquatic organisms. Hydraulic model tests at Alden Research Laboratories resulted in the development of an efficient jet injection system which gives maximum rapid mixing effect. A jet of from 5 fps to 8 fps at the end of a long canal introduces the effluent throughout the entire depth of the water column in the reservoir. This system was much superior to the use of a short discharge canal which utilized a weir to release the warm water at the surface at low velocity. The studies of isotherms as related to results of 3 yr of detailed biological data indicated that at the critical seasons an increase of ambient water temperature of 13○ F introduced as a jet was satisfactory in meeting the temperature requirements of the fishes and associated organisms. An open circulating system is to be used with mechanical cooling towers to reduce the effluent from 21○ F to 13○F above ambient. Low approach velocities at the cooling water screen structures are recommended to safeguard screenable fishes.